updated the Iterator interface docs for methods Key() and Value()

types.go

@@ -139,12 +139,14 @@ type Iterator interface {
 	// Key returns the key of the current key/value pair, or nil if done.
 	// The caller should not modify the contents of the returned slice, and
 	// its contents may change on the next call to any 'seeks method'.
+	// Instead, the caller should make a copy and work on the copy.
 	Key() (key []byte)
 
 	// Value returns the value at the current position. Panics if the iterator is invalid.
 	// Value returns the value of the current key/value pair, or nil if done.
 	// The caller should not modify the contents of the returned slice, and
 	// its contents may change on the next call to any 'seeks method'.
+	// Instead, the caller should make a copy and work on the copy.
 	Value() (value []byte)
 
 	// Error returns the last error encountered by the iterator, if any.